Lassa fever virus is a member of the Arenaviridae, which is responsible for substantial morbidity and mortality in sub-Saharan West Africa. Lassa virus persists in nature in its rodent host, Mastomys natalensis, which excretes high titers of virus in the urine. Infection of man usually occurs by direct exposure to rodent excretia, although nosocomial infections also occur, and on numerous occasions disease has been exported to countries outside of Africa. Available evidence from animal models suggest that recovery from Lassa virus infections is dependent primarily on cellular immune mechanisms, and neutralizing antibody titers only appear late in the convalescent stages of disease. In order to gain a better understanding of the specificity and mechanisms of this protective cell mediated immune response, it is proposed to utilize recently developed peptide epitope predictive strategies to predict the human MHC class l restricted T-cell epitopes recognized on the Lassa virus glycoprotein and nucleocapsid protein genes by Lassa seropositive individuals bearing multiple alleles of MHC class I. Epitopes will be predicted based on algorithms which predict supermotifs bearing requisite position 2 and 9 anchor residues and secondary hydrophobic pocket residues for HLA-A1, A2, A3, A11, A24, A33, B7 and B44, which in the aggregate are represented in more than 90% of blacks and caucasians. Peptides containing predicted motifs and supermotifs will be synthesized by multiple peptide synthesis strategy and tested for binding activity on class molecules. Peptides demonstrating binding activity will be applied to target cells and cytotoxic activity of CTL expanded from seropositive individuals will be assessed by 51Cr release assay to positively identify functional CTL epitopes. It is anticipated that this analysis will provide a clear picture of the specificity of human CTL activity directed against Lassa virus and aid in targeting protective vaccination strategies.